Nanoparticles can exhibit physical and chemical properties that sometimes differ significantly from those observed in the bulk material. This is particularly true for copper nanoparticles, which can exhibit a significantly reduced melting point relative to that of bulk copper. In particular, copper nanoparticles having a narrow size range and nanoparticle sizes of less than about 20 nm fuse together at much lower temperatures and pressures than do larger copper nanoparticles or bulk copper.
Although copper nanoparticles are of significant interest due, inter alia, to the widespread industrial use of bulk copper, the formation of monodisperse copper nanoparticles remains synthetically challenging. Copper nanoparticles having a narrow size range that are less than about 20 nm in size have been particularly difficult to synthesize. Solution-based chemical reduction methods have typically produced nanoparticles having irregular shape, wide size ranges, and/or nanoparticle sizes that are much larger than 20 nm. Furthermore, many methods for synthesizing copper nanoparticles are not readily amenable to scale up.
Only a limited number of scalable processes are available for producing monodisperse copper nanoparticles having small nanoparticle sizes. One readily scalable procedure for synthesizing copper nanoparticles having nanoparticle sizes below about 20 nm, more particularly below about 10 nm, involves heating a copper salt solution, a bidentate diamine (e.g., a N,N′-dialkylethylenediamine), and one or more C6-C18 alkylamines. Copper nanoparticles produced by this method have a fusion temperature of less than about 200° C., with the fusion temperature decreasing as a function of nanoparticle size. Copper nanoparticles in this size range have also been produced by the reduction of a copper salt in the presence of ascorbic acid. Although copper nanoparticles in this size range can be isolated, characterized and utilized, they do have a somewhat limited shelf life. Further, rapid oxidation of copper can take place if the copper nanoparticles are incompletely fused after heating.
In view of the foregoing, facile utilization of copper nanoparticles having nanoparticle sizes of less than about 20 nm would be of substantial benefit in the art. The present invention satisfies this need and provides related advantages as well.